WASHINGTON -- In a fresh sign of Republican turmoil, Texas Sen. Ted Cruz, tea party groups and losing challenger Chris McDaniel all backed an investigation Tuesday into the June 24 Senate primary runoff in Mississippi, without offering evidence of alleged voter fraud they cited in the bitterly fought race.

One day after Sen. Thad Cochran was certified the primary winner by the state Republican Party, the denunciations of the 76-year-old lawmaker were particularly vituperative.

The Tea Party Leadership Fund, which claims a membership of 25,000, referred to him as "scum." An appeal sent by the Madison Project under the name of former Kansas Republican Rep. Jim Ryun labelled the six-term veteran corrupt.

Cruz, a tea party favorite known for his clashes with the GOP establishment, told reporters that allies of Cochran had run racially offensive ads aimed at persuading black voters to cast ballots in the Republican primary.

On Monday evening, he said on a conservative radio program, "The Mark Levin Show," that "even more troubling is in the past week or so we've seen serious allegations of voter fraud. ... These allegations need to be vigorously investigated and anyone involved in criminal conduct should be prosecuted."

McDaniel's campaign followed with a statement of its own that cited Cruz' remarks, underscoring the Texas senator's standing among tea party supporters and potential donors.

The controversy came midway through a campaign year in which the Republican Party nationally has fought hard to prevent a replay of the 2010 and 2012 election campaigns in which they failed to win a Senate majority. In both cases, tea party-backed candidates won Senate primaries over more established and steadier politicians, only to prove unelectable in the fall in races that had appeared winnable for the GOP.

Favored candidates of tea party groups have lost in several primaries in other states. The Mississippi runoff was the bitterest pill to date for them to swallow, since McDaniel led Cochran in the initial June 3 primary. Because neither man won a majority in the three-way race, a runoff was held.

Cochran is the overwhelming favorite to win re-election this fall against former Democratic Rep. Travis Childers. His schedule for the day included an appearance at what the National Republican Senatorial Committee billed as a thank-you reception. Donations were not required but are generally welcomed at events at the campaign organization's headquarters a few blocks from the Capitol.

At issue in the runoff is an unknown number of votes cast by Mississippians who voted earlier this year in the Democratic primary. Under state law, they are prohibited from doing both. McDaniel and his allies say they suspect they did so anyway, by the thousands, and note an appeal by Cochran and his supporters to black voters in the final days of the race.

At the same time, neither they nor Cruz nor tea party groups have offered documentation of their claims. Nor do McDaniel and his supporters routinely note that state law generally permits voters of one party to cast ballots in primaries of the other. Mississippi voters do not register by party.

Cochran's campaign said a partial statewide review showed 234 questionable ballots had been cast. "The numbers contained in this review are drastically lower than the wild claims made by the McDaniel campaign," it said.

Results certified by the state party and submitted to the secretary of state's office on Monday show Cochran defeated McDaniel by 7,667 votes in the June 24 runoff. Cochran received 194,932 votes and McDaniel received 187,265.